Novel class of superlattice materials and superlattice precursors, and method for their manufacture and use

ABSTRACT

The present disclosure concerns novel materials comprising at least two crystalline materials. In certain embodiments, at least one of the crystalline materials is a diffusion barrier, and at least one material has a high power factor. The disclosed materials are particularly useful as superlattices, particularly thermoelectric superlattices, and superlattice precursors. A method for synthesizing such superlattices is provided. An embodiment of the method includes using Modulated Elemental Reactants (MER) to deposit layers of superlattice precursor materials, followed by annealing to yield bulk superlattice materials.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of the earlier filing date of U.S.provisional application No. 60/398,953, filed Jul. 26, 2002, which isincorporated herein by reference.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made in part using funds provided by NationalSciences Foundation Grant Nos. DMR 9813726, DMR 0103409, and DGE0114419. The United States government may have certain rights in thisinvention.

FIELD

This application concerns a novel composition of matter, particularly asuperlattice composition, even more particularly a thermoelectricsuperlattice composition, comprising two crystalline materials, at leastone of which functions as a diffusion barrier, and embodiments of amethod for their manufacture and use.

BACKGROUND

Thermoelectric materials can directly convert thermal energy intoelectricity, and conversely, can convert electricity into thermalenergy. Thermoelectric materials can be used for many differentapplications, which typically fall into two general categories: powergeneration and cooling devices. Researchers have proposed usingthermoelectric materials for such diverse applications as convertingwaste heat energy into electrical power in automobiles and internallyspot-cooling microelectronic components. Voss, D. Technology Review,April 2002, 29. Unfortunately, known thermoelectric materials currentlyhave limited utility in devices because of their efficiency.

Two effects are known to govern thermoelectric behavior: the Peltiereffect and the Seebeck effect. The Seebeck effect is demonstrated whentwo conducting materials are joined at two different places, and eachjunction is kept at a different temperature. A potential differenceforms between the two materials and current flows between them, asillustrated in FIG. 1, where the arrows indicate current flow.

The Peltier effect is the converse of the Seebeck effect. If a currentis forced to flow through two materials that are connected at twoconstant temperature junctions, the junctions will absorb or releaseheat as current flows from one material to another (see FIG. 2). CRCHandbook of Thermoelectric; Rowe, D. M., Ed.; CRC Press: New York, 1995.

The energy transfer efficiency of such devices is related to the figureof merit. For a pair of materials, the figure of merit is defined byFormula 1: $\begin{matrix}{Z = \left\lbrack \frac{S_{1} - S_{2}}{\left( {\rho_{1}\kappa_{1}} \right)^{\frac{1}{2}} + \left( {\rho_{2}\kappa_{2}} \right)^{\frac{1}{2}}} \right\rbrack^{2}} & {{Formula}\quad 1}\end{matrix}$where Z is the figure of merit, S is the Seebeck coefficient, K is thethermal conductivity, and ρ is the resistivity. The figure of merit fora single material is given by Formula 2: $\begin{matrix}{{ZT} = {\left( \frac{\sigma\quad S^{2}}{\kappa} \right)T}} & {{Formula}\quad 2}\end{matrix}$where Z is defined as the figure of merit of a thermoelectric material,a is the electrical conductivity, S is the Seebeck coefficient, T is thetemperature in Kelvin, and κ is the total thermal conductivity,including both electronic and lattice contributions.

In Formula 2, the figure of merit (Z) is maximized when the electricalproperties of the material are maximized and the thermal conductivity ofthe material is minimized. Most metals have small Seebeck- coefficientsand high electrical conductivities, but also have large thermalconductivities. Most insulators have high Seebeck coefficients and lowthermal conductivities, but very low electrical conductivities.

Good thermoelectric materials ideally have properties from both types ofmaterials. These materials typically have a high power factor value,ο•S²T, which is calculated from the numerator of the unitless figure ofmerit equation in Formula 2. Materials Research Society: SymposiumProceedings, Vol. 691 Thermoelectric Materials 2001 —Research andApplications. Editors: George S. Nolas, David C. Johnson, David G.Mandrus, Materials Research Society, 2002 Overview of Various Strategiesand Promising New Bulk Materials for Potential ThermoelectricApplications; pp. 3-14.

With reference to Formula 2, where thermal conductivity (κ) is reduced,the figure of merit is increased. A method for reducing the thermalconductivity of materials that is exploited herein involves usingsuperlattices to reduce thermal conductivity and therefore increase thefigure of merit.

Superlattice materials are of interest as thermoelectric materials.Bi₂Te₃/Sb₂Te₃ superlattices have been synthesized in thin film form andthe thermoelectric properties of these thin films were evaluated.Venkatasubramanian, R.; Colpitts, T. In Thermoelectric Materials—NewDirections and Approaches; Tritt, T. M., Kanatzidis, M. G., Hylan B.Lyon, J., Mahan, G. D., Eds.; Materials Research Society: San Francisco,Calif., 1997; Vol. 478, pp. 73-84, which is incorporated herein byreference. These films were prepared using epitaxial metallorganicchemical vapor deposition (MOCVD). Superlattices with a repeat distance(i.e., the thickness of the superlattice repeat unit), of 10 to 100 Åwere found to have a three-to four-factor reduction in thermalconductivity compared to the corresponding bulk alloy. From this, theunitless figure of merit (ZT) was calculated to be approximately 2 at300K. This is significantly higher than ZT≈1 for the p-typeBi_(0.5)Sb_(1.5)Te₃ alloy and ZT≈0.9 for the n-typeBi₂Te_(2.85)Se_(0.15) alloys, which are commonly used in thermoelectricdevices. These results indicated a maximum efficiency was achieved at asuperlattice repeat thickness of approximately 50 to 70 Å.Venkatasubramanian, R.; Colpitts, T.; Watko, E.; Lamvik, M. Journal ofCrystal Growth 1997, 170, 817-821.

Epitaxial MOCVD is not suitable for making bulk superlattices. Sincethermoelectric devices require bulk material, methods for producing suchbulk materials are needed. Moreover, the properties of superlatticematerials should be determined on the bulk materials to ensure that thethermoelectric properties are retained in bulk samples.

SUMMARY

The present disclosure concerns a novel class of material, particularlythermoelectric superlattices, and a method for synthesizing suchsuperlattices. The superlattices are comprised of layers ofthermoelectric materials. The present superlattices typically include atleast two different materials, and can include three or more differentmaterials operatively positioned relative to one another to define asuperlattice, such as by being stacked on one another. The superlatticetypically is formed on a substrate, such as silicon, silicon nitride,glass, plastics, insulating oxides, semiconductor materials, quartz,mica, metals, and combinations thereof. The different materials of thesuperlattice each form a substantially discrete superlattice componentlayer.

Generally, the superlattice includes a first material, typically havinga high power factor, and a second material that functions as a diffusionbarrier. Each layer can include elements, such as antimony, bismuth,hafnium, lead, selenium, tellurium, titanium, zirconium and combinationsthereof. Particular embodiments of superlattice layers can comprise anymaterial, or combinations of materials, which are described as metalchalcogenides. Typical examples of metal chalcogenides include thosehaving the general formula Bi,Sb_(2-x)Se_(y)Te_(3-y), orPbSe_(z)Te_(1-z), where 0≦x≦2, 0≦y≦3, and 0≦z≦1. Certain superlatticelayers will include materials fitting one formula or both formulas. Forexample a superlattice layer can include a first material such as onesatisfying a formula provided above, for example Bi₂Te₃, or such amaterial alloyed with a second material, e.g. a material having aformula provided above, e.g. PbSe_(z)Te_(1-z) where 0≦z≦1. Similarly,certain superlattices will include different materials fitting oneformula, or both formulas. Superlattices can, for example, include amaterial selected from the group consisting of Bi₂Te₃, Sb₂Te₃, Bi₂Se₃,Sb₂Se₃, PbSe, PbTe, alloys thereof, and combinations thereof. Forexample, without limitation, a superlattice can include both Bi₂Te₃ andSb₂Te₃, and a superlattice can include both Bi₂Te₃ and PbTe.

The superlattice typically includes at least one layer that functions asa diffusion barrier. The diffusion barrier layer serves to maintain theintegrity of the superlattice layers, such that they remainsubstantially discrete and layer interdiffusion is minimized. Thediffusion barrier layer can comprise any material or combinations ofmaterials capable of functioning as a barrier material. Particularembodiments used barriers having the formula ASe_(z)Te_(2-z), where Aincludes, without limitation, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, andcombinations thereof, and 0≦z≦2. By way of example and withoutlimitation, diffusion barrier materials according to the formulaASe_(z)Te_(2-z), include TiTe₂, HfTe₂, ZrTe₂, TiSe₂, HfSe₂, ZrSe₂, VSe₂,NbSe₂, TaSe₂, CrSe₂, MoSe₂, MoSe₂, WSe₂, VTe₂, NbTe₂, TaTe₂, CrTe₂,MoTe₂, WTe₂, alloys thereof, and combinations thereof. Mixed aniondiffusion barrier materials also can be used, such as TiSeTe and HfSeTe.These materials are referred to as mixed anion materials because theyeach have two different chalcogens. A particular example diffusion layeruseful for forming superlattices including Bi₂Te₃, Sb₂Te₃, or both, isTiTe₂. Exemplary superlattices can be represented as a whole by therepeating unit [(Bi₂Te₃)_(x)(TiTe₂)_(y))], where x and y refer to thenumber of contiguous repeat layers for Bi₂Te₃ and TiTe₂, respectively.

A method for synthesizing individual superlattice component layers alsois disclosed. Generally, the method involves synthesizing repeatingsuperlattice component layers, thereby forming a superlattice. Novelmaterials can be made by the particular working embodiments describedherein or by using known or hereafter developed synthetic methods.Described working embodiments of the method included preparingsuperlattice layers by modulated elemental reactants (MER).Superlattices are assembled according to this method by depositingstoichiometrically correct amounts of elements necessary to form thedesired stacked component compounds. After the appropriatestoichiometric amounts of materials are deposited, the thin filmprecursors are annealed to form the desired superlattice componentlayers. Alternatively, the desired compounds, such as Bi₂Te₃ and TiTe₂,can be directly deposited on a substrate as thin film precursors.Subsequent layers then can be deposited on the first, such that arepeating superlattice structure is formed.

Typically MER yields flakes or chips of superlattice materials.Accordingly, one aspect of the method uses hot isostatic pressing toprepare ingots of bulk superlattices, which are more useful for largescale devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing depicting the Seebeck effect.

FIG. 2 is a schematic drawing depicting the Peltier effect.

FIG. 3 shows X-ray diffraction patterns as a function of annealingtemperature for a forming [(Bi₂Te₃)₃(TiTe₂)₃] superlattice.

FIG. 4 is a low angle XRR pattern of a typical Bi—Te precursor film.

FIG. 5 is a low angle XRR pattern of a representative superlatticecomprising Bi₂Te₃/TiTe₂.

FIG. 6 is a representative low angle XRR pattern for Bi₂Te₃/TiTe₂superlattice precursors.

FIG. 7 is an XRD pattern obtained for the Bi₂Te₃/TiTe₂ superlatticesystem, with each numbered peak indicating a Bragg reflection.

FIG. 8 is a plot of actual repeat layer thickness vs. intended bismuththickness, where the y-axis intercept is actual thickness of selenium.

FIG. 9A illustrates a calibration method for MER deposition.

FIG. 9B illustrates additional steps of the calibration method of FIG.9A.

FIG. 10 is a graph of actual repeat layer thickness (Å) vs. intendedthickness (Å) for calibrating the deposition of TiTe₂.

FIG. 11 is a plot of the atomic percent of bismuth versus actualthickness of bismuth.

FIG. 12 is a low angle XRD pattern (Log CPS vs. 2θ) for a Bi₂Te₃/HfTe₂superlattice.

FIG. 13 shows overlaid X-ray diffraction patterns of five isomericsuperlattices having the formula [(Bi₂Te₃)_(x)(TiTe₂)_(y)].

FIG. 14 is a graph of thermal conductivity (measured perpendicular tosuperlattice orientation) versus temperature for a bulk pellet ofsuperlattice material having the formula [(Bi₂Te₃)₆(TiTe₂)₂].

FIG. 15 is a graph of thermal conductivity (measured parallel tosuperlattice orientation) versus temperature for a bulk pellet ofsuperlattice material having the formula [(Bi₂Te₃)₆(TiTe₂)₂].

FIG. 16 is a graph comparing the lattice thermal conductivity (κ_(L)) ofbulk Bi₂Te₃, and two bulk superlattice materials having the formula[(Bi₂Te₃)₆(TiTe₂)₂].

DETAILED DESCRIPTION

Superlattices are difficult to make by traditional methods. Traditionalsynthetic techniques do not provide the necessary order to makesuperlattices. The inability to control the local composition,diffusion, and crystallization has led to the development of newsynthetic techniques. Some of the techniques that have been used tocreate these highly ordered products include pulsed-laser deposition(PLD) and molecular beam epitaxy (MBE), and epitaxial metallorganicchemical vapor deposition (MOCVD). These methods have a significantdrawback in that only a small amount of material can be made using thesemethods. This places a significant restriction on synthesizing bulkamounts for experimental determination and device manufacture.

One embodiment of the present method for superlattice synthesis is knownas Modulated Material Reactants or Modulated Elemental Reactants (MER),which involves evaporating elements or compounds in a vacuum depositionchamber. According to the technique, stoichiometrically accurate amountsof the desired elements or compounds are deposited, followed by anannealing step to form the desired material. The annealing conditionsmay be calibrated by monitoring with XRD for Bragg reflections. See, forexample, FIG. 3, which provides the results of an annealing study for aBi₂Te₃/TiTe₂ superlattice. The material evaporation is performed withthe vacuum deposition chamber at low pressure, typically less than about10⁻⁴ torr. More typically, material evaporation is performed at evenlower pressures. Working examples used ultra-high vacuum, which istypically less than about 10⁻⁶ torr. However the evaporation may beperformed under any conditions sufficient to vaporize a particularmaterial. The evaporation method is specific to the material. In thecase of high temperature evaporation, electron beam guns may be used toheat the metal. With lower temperature evaporation, effusion and/orKnudsen cells may be used to heat the source metal to its evaporationtemperature under UHV.

According to disclosed embodiments of the synthetic method, desiredmaterials are deposited on a substrate using, for example MER. Thesubstrate material may be selected from the group consisting of silicon,silicon nitride, glass, plastics, insulating oxides, semiconductormaterials, quartz, mica, metals, and combinations thereof. Workingembodiments used a substrate comprising a silicon wafer.

Such wafers can include other materials deposited thereon. Theseadditional materials can be used, for example, to provide electricalconnection between a thermoelectric superlattice and other devicecomponents. Working embodiments used wafers spin-coated with a thinlayer of polymethylmethacrylate (PMMA). Other suitable insulating orresist materials that can be used in place of, or in combination withPMMA, are other polymeric materials, such as vinyl-derived or polyethermaterials, such as polystyrene and polyethylene oxide polymers. Inworking embodiments, the wafer materials were simply used as a substratefor synthesizing materials, which were removed from the substrate aftersynthesis. Accordingly, after a deposition, the superlattice materialcan be removed from the substrate by dissolving the PMMA in acetone. Themetal sample is then collected by vacuum filtration, and characterizedby X-ray reflection (XRR)/diffraction (XRD) studies.

The MER apparatus uses pneumatically activated shutters to select thedeposition source. The shutters block the path of material flux, whilequartz crystal monitors are used to monitor and control the depositionrate of the sources. Control of the flux allows precise control of thethickness of the precursor elements deposited, which leads to astoichiometrically correct compound.

Using the MER technique described above, kinetically stable and highlyordered systems, such as superlattices, can be made from thin-filmprecursors. The samples can then be characterized by differentialscanning calorimetry (DSC) to determine the crystallization temperature,as well as the relative nucleation energy as a function of composition.They also can be examined with XRR and XRD to analyze the thickness ofdeposited layers, as well as the crystalline lattice spacings that formafter crystallization. The composition of the samples typically ismonitored with Electron Probe Micro Analysis (EPMA).

Thermoelectric superlattices can be prepared according to embodiments ofthe present method from materials that have good electrical conductivityand low thermal conductivity. Semiconducting materials, such as smallbandgap semiconductors, satisfy both requirements.

One effect that has been observed in superlattices is decreased thermalconductivity in both parallel and perpendicular directions. Chen, et al.reported reduced measured thermal conductivity both in-plane andcross-plane in Si/Ge superlattices. Borca-Tasciuc, T.; Song, D.; Liu, J.L.; Chen, G.; Wang, K. L.; Sun, X.; Dresselhaus, M. S.; Radetic, T.;Gronsky, R. In Materials Research Society Symposium Proceedings, 1999;Vol. 545, pp. 473-478. Wu, et al. also observed reduced thermalconductivity in the parallel and perpendicular directions in GaAs/AlGaAssuperlattices. Both electronic and lattice effects contribute to thethermal conductivity term κ in Formula 2. The Wiedemann-Franz law statesthat the electronic contribution to the thermal conductivity isproportional to the electrical conductivity at a given temperature. See,CRC Handbook of Thermoelectrics; Rowe, D. M., Ed.; CRC Press: New York,1995. To change the electronic contribution, it would be necessary tochange the electrical conductivity. The lattice contribution to thermalconductivity is associated with phonons traveling through the lattice,and adjustments to this physical property may yield the best opportunityto minimize thermal conductivity without affecting a material'selectrical properties.

Two main approaches have been used to reduce the lattice thermalconductivity. The first approach involves using rattling atoms torefract phonons. Rattling atoms are heavy atoms loosely bound in voidswithin a crystalline lattice. Rattling atoms move more freely than thestrongly bound atoms of the crystalline lattice, and thus can refractphonons traveling through the material. Common rattling-atom-basedmaterials are skutterudites and clathrates.

The second approach to reduce lattice thermal conductivity involvesusing superlattices, which is the approach adopted herein. Insuperlattice structures, several mechanisms responsible for minimizingthermal transport are theorized. The first mechanism by which thermaltransport is minimized involves lowering the minimum phonon energiesrequired to produce Umklapp scattering processes relative to that of analloy including the component superlattice materials. Other factors arephonon wave reflection at superlattice interfaces when the phononwavelength fulfills the Bragg condition, or when an acoustic mismatch isencountered. Venkatasubramanian, R.; Colpitts, R. In ThermoelectricMaterials—New Directions and Approaches; Tritt, T. M.; Kanatzidis, M.G.; Hylan B.; Lyon, J.; Mahan, G. D., Eds.; Materials Research Society:San Francisco, Calif., 1997; Vol. 478, pp. 73-84; Venkatasubramanian,R.; Slivola, E.; Colpitts, T.; O'Quinn, B. Nature, 2001, 413, 597-602.

Superlattices are defined as having repeating structure, where therepeating layers are crystallographically oriented as detected bysuperlattice Bragg reflections, and which have at least four componentlayers. Thus, superlattices comprise a repeat unit having at least twocomponent layers, with the repeating unit repeating at least once. Thedifferent materials in the repeat unit scatter phonons. Since theelectrical conductivity and Seebeck coefficient are not independent, itis very difficult to change these terms experimentally to increaseefficiency. However, the lattice thermal conductivity is one variablethat can be adjusted to optimize Z.

If the transition between the two materials is abrupt, thermalconductivity is reduced. To ensure that the transition is abrupt,typically each material extends for whole numbers of unit cells. Thepresence of abrupt transitions can be readily confirmed by observationof characteristic Bragg reflections in the X-ray diffraction pattern.The MER method discussed above provided the precision necessary todeposit the correct stoichiometric amounts of materials to correspond tointeger numbers of unit cells. Thus, in working examples, one or moreunit cells of a first material were prepared on one or more unit cellsof a second material, thereby forming a new, superlattice repeat unitincluding both materials. The superlattice repeat unit was then repeatedor stacked one or more times to form the superlattice.

Good thermoelectric materials ideally have a high power factor value.High power factor typically refers to a power factor of at least about0.1 Wm⁻¹K⁻¹. High power factors that have been observed typically rangefrom 0.1 to 5 Wm⁻¹K⁻¹; however, materials having higher power factorsshould be able to exist. Examples of materials having high power factorsinclude, without limitation, Bi₂Te₃, Sb₂Te₃, CoSb₃, HfNiSn and PbTe.

Component layers of the superlattice may be any thickness that yieldsgood thermoelectric properties for the bulk superlattice. Typically theminimum thickness is about the thickness of a unit cell for theparticular material, and the maximum thickness is such that the materialretains useful thermoelectric properties. Useful thermoelectricmaterials typically have a high figure of merit. With reference toFormula 2, a high figure of merit typically yields a ZT value of atleast about 0.5 at the temperature of desired use. Typically eachcomponent layer has a thickness of from about 3 Å, to several hundredangstroms. Typically a superlattice repeat unit, made from the componentlayers, can have a lower thickness limit of about 6 Å, and a large upperthickness limit, such as at least about 500 Å, more typically betweenabout 50 Å and about 100 Å, with working embodiments having superlatticerepeat unit thicknesses of from about 40 Å to about 50 Å. The upperthickness limit is material-dependent.

According to the MER technique, superlattices are prepared using thinfilm, multilayer precursors. The thin film precursors are provided asthe individual, various elements needed for a final product, and aredeposited sequentially on a substrate. Various elements can be depositedusing MER, including elements that may be defined as metals, rare earthmetals or metalloids. Typical examples of elements useful for formingsuperlattices deposited by MER include antimony, bismuth, hafnium, lead,selenium, tellurium. titanium, zirconium and combinations thereof.

Embodiments of the presently disclosed superlattices typically includeat least a first layer comprising Bi_(x)Sb_(2-x)Se_(y)Te_(3-y), orPbSe_(z)Te_(1-z), where 0≦x≦2, 0≦y≦3, and 0≦z≦1. Typically the firstlayer includes a material having a high power factor.

The disclosed superlattices also usually include a second layer, whichfunctions as a diffusion barrier. Such diffusion barriers are designedto include materials that have slower interdiffusion rates. Embodimentsof diffusion barriers for use in the present superlattices includecompounds such as selenides and tellurides. Typical embodiments includecompounds having a general formula ASe_(z)Te_(2-z), where A can be Ti,Zr, Hf. V, Nb, Ta, Cr, Mo, W., and combinations thereof, and 0≦z≦2.Without limitation to theory, it currently is believed that materialspreferred for use as diffusion barriers have strong atomic bonds, andare substantially immiscible in and have a similar crystal structure to,the other superlattice component(s) materials. For example, TiTe₂ hasstrong atomic bonds, is immiscible in, and has a similar crystalstructure to an exemplary superlattice material used for the firstlayer, Bi₂Te₃. Thus, certain working embodiments have used TiTe₂ as adiffusion barrier. A second diffusion barrier used in workingembodiments with Bi₂Te₃ is HfTe₂, which corresponds to the formulaASe_(z)Te_(2-x), where z is 0 and A is hafnium.

Certain embodiments include a third layer comprising a differentmaterial than in the first layer, but also having the formulaBi_(x)Sb_(2-x)Se_(y)Te_(3-y), or PbSe_(z)Te_(1-z), where 0≦x≦2, 0≦y≦3,and 0≦z≦1. Typical embodiments have a layer including a material wherey=0, to provide Bi_(x)Sb_(2-x)Te_(3-y) where 0≦x≦2. Working embodimentsused superlattice layers including Bi₂Te₃, which corresponds to theformula Bi_(x)Sb_(2-x)Se_(y)Te_(3-y) where x is 2 and y is zero.

Without limitation to theory, phonon scattering is thought to be moreprevalent in superlattices that have sharp interfaces betweencrystalline structures. Thus, superlattices having such sharp interfacesshould exhibit reduced thermal conductivity. To ensure an abruptelectronic density change between superlattice layers, the layers shouldremain substantially distinct. The diffusion barrier is included indisclosed superlattices to ensure that interdiffusion of superlatticematerials is minimized. To produce superlattices having the desired,substantially distinct layers, a diffusion barrier can be includedbetween each adjacent layer, only between certain layers, or perhapsbetween only one pair of layers. The diffusion layer may be deposited byany suitable technique. For example, a diffusion layer can be depositedby MER.

Until now, Bi₂Te3/Sb₂Te₃ superlattices have been synthesized only asthin films. The calculation of ZT for thin films is not straightforward.The method for measuring the thermal conductivity of thin films is knownas the 3ω technique. The details of this technique are explainedelsewhere. Borca-Tasciuc, T.; Song, D.; Liu, J. L.; Chen, G.; Wang, K.L.; Sun, X.; Dresselhaus, M. S.; Radetic, T.; Gronsky, R. In MaterialsResearch Society Symposium Proceedings, 1999; Vol. 545, pp. 473-478;Cahill, D. Rev. Sci. Instrum. 1990, 61, 802-808. Because superlatticesare intended for bulk use to make high Z thermoelectric materials, thereis cause for concern that the thermal conductivity measurements obtainedfrom thin films will not accurately depict the thermal conductivity forthe equivalent bulk superlattice structures. In bulk, other factors,such as grain boundaries and orientation of the various particles in athree-dimensional space, can affect the thermoelectric properties. Inprevious reports of thin film Bi₂Te₃/Sb₂Te₃ superlattices, calculationsimply that the superlattices could offer a three to four-fold decreasein the thermal conductivity over bulk Bi_(0.4)Sb_(1.6)Te₃ alloys.Venkatasubramanian, R.; Colpitts, T. In Thermoelectric Materials—NewDirections and Approaches; Tritt, T. M., Kanatzidis, M. G., Hylan B.Lyon, J., Mahan, G. D., Eds.; Materials Research Society: San Francisco,Calif., 1997; Vol. 478, pp. 73-84. This claim remains to be tested onbulk superlattice samples.

Bulk amounts of superlattices with varying superlattice repeat distanceshave been prepared, verifying that the MER method can be used tosynthesize these superlattices with sharp interfaces. The properties ofthe material are then tested to determine the ZT of the material in thebulk. An optimal superlattice thickness for the bulk material can bedetermined that achieves a preferred or maximum figure of merit, ZT.

MER evaporation techniques have been developed to deposit elementallayers on a substrate. Because thin elemental layers can be deposited bythe method, interdiffusion and crystallization can occur at very lowtemperatures. Thus, since the elements do not have to travel very far tofind their respective elemental matches, the superlattices might begrown easily. Noh, M. In The synthesis and characterization ofcrystalline superlattices ((TiSe₂)(_(l))(NbSe₍₂₎)_((m)))_((n)): A newthin-film growth technique using multilayer reactants. Ph.D. Thesis,Chemistry; University of Oregon: Eugene, Oreg., 1997, UMI No. 9818734.

A compound or compounds having a crystal structure similar to Bi₂Te₃ andSb₂Te₃ were deemed preferable for the interdiffusion barrier. It is alsothought that diffusion barrier materials having strong interatomic bondsare more effective due to slower diffusion rates. Thus, TiTe₂ and HfTe₂were selected for use as diffusion barrier materials for certain workingembodiments, because (1) these compounds have a layered crystalstructure comprising hexagonal sheets, much like bismuth telluride andantimony telluride, (2) titanium-tellurium and hafnium-tellurium bondsare strong, and (3) the compounds are insoluble in bismuth telluride.Oftedal, I. Z. physik. Chem. 1928, 134, 301-310. All three of thesefeatures are believed to minimize interdiffusion the diffusion barrierwith the material having a high power factor, which preserves the sharpinterfaces between the different materials. Thus, by minimizinginterdiffusion, the diffusion barrier reduces thermal conductivity.

Binary (two different materials) superlattices, including Bi₂Te₃ andTiTe₂ and superlattices comprising Bi₂Te₃ and HfTe₂ component layers,were prepared. The synthesis of the Bi₂Te₃/TiTe₂ superlatticesillustrates the use of diffusion barriers to synthesize superlattices bythe MER technique.

To synthesize the Bi₂Te₃/TiTe₂ superlattice the deposition of elementalBi and Te layers was first calibrated. Since the target compound wasBi₂Te₃, the relative thicknesses between the Bi and Te were correlatedto provide the desired stoichiometry. For this application, Bithicknesses ranging from about 24 Å to about 33 Å corresponded to anatomic % of from about 30% to about 40%. Similarly, Te thickness ofabout 51 Å provided, with varying amounts of Bi, from about 60 to about70 atomic % Te. During this calibration, a series of Bi—Te samples weredeposited. The Te thicknesses were held constant while the Bithicknesses were varied. Table 1 lists the samples that were used tocalibrate the Bi—Te system's stoichiometry. The composition data inTable 1 was determined from EPMA performed on floated flakes ofmaterial, which determines the ratio of Bi and Te atoms independent ofstructure. TABLE 1 Calibration of Bi—Te deposition. Intended IntendedThickness Bi Thickness Te (Å) (Å) Atomic % Bi Atomic % Te 24 51 31.768.3 27 51 37.6 62.4 30 51 38.1 61.9 33 51 39.7 60.3

Layer thickness also was optimized during the system calibration so thateach layer ended on a unit cell, thus providing a van der Waals gap(VWG). Low angle, X-ray reflectivity experiments can be used to evaluatelayer thickness. However, with reference to FIG. 4, which shows atypical low angle reflectivity pattern for the Bi-Te system, the onlypeaks that appear in the low angle are front surface to back surfacereflections. In an ideal reflection pattern of a multi-element layeredfilm, a Bragg peak appears at a higher 2 theta value that corroboratesthe total thickness data (see FIG. 6). Much like the Bi₂Te₃ system, theTiTe₂ system was calibrated for stoichiometry and thickness.

FIG. 5 illustrates a representative superlattice from an initialpreparation of Bi₂Te_(3/)TiTe₂. Preliminary calibrations from the twobinary systems were sufficient to enable formation of a superlattice,which is evidenced by the higher order Bragg components observed in thespectrum of FIG. 5.

The crystal structures of Bi₂Te₃ and TiTe₂ are similar in that they areboth layered hexagonal systems that contain VWGs in between therepeating units. When synthesizing superlattices with crystal structuressuch as these, each material preferably ends on its VWG. With thisstrategy the correct stoichiometry, and also the correct amount of eachmaterial, preferably is deposited. The superlattice spacing betweenrepeating units can be calculated using Bragg's law and, for example,the peaks from FIG. 5. In a superlattice having regular repeats, all ofthe peaks represent the same thickness.

FIG. 6 shows a representative low angle reflection pattern for anas-deposited Bi₂Te₃/TiTe₂ superlattice precursor sample. Bragg peaks canbe seen describing the repeat layer thickness. Since these Bragg peaksonly describe the total repeat of the Bi—Te layers as well as the Ti—Telayers, the individual thickness of the Bi—Te regions as well as theTi—Te regions cannot be determined from one sample.

However, a technique was developed to determine the thicknesses of allof the regions. In this technique, a series of superlattice precursorswere deposited in which one superlattice component was varied while theother was held constant. For example, a working embodiment held theBi—Te layers constant and varied the Ti—Te layers. By using the lowangle X-ray reflection data, the various superlattice repeat distancescan be plotted as a function of intended Ti—Te thickness. The slope ofthe resulting line provides the thickness of the Ti—Te layers, and they-intercept provides the Bi—Te thickness. Once the thickness values havebeen obtained for the different layers, corrections in the monitoredthickness can be made for the next deposition set. The process can thenbe repeated with the calibrated values.

Once the optimal thicknesses have been achieved, the result is an idealBi₂Te₃/TiTe₂ superlattice. FIG. 7 shows the high angle diffractionpattern of the Bi₂Te₃/TiTe₂ superlattice having both optimalstoichiometry and thickness. The superlattice repeat distance can becalculated from the diffraction peaks in FIG. 7. Once the thicknessesare calibrated, the uncertainty of the calculated superlattice thicknessbecomes very narrow so that superlattices having precise stochiometriescan be prepared.

EXAMPLES

The following examples are provided to illustrate certain particularembodiments of the disclosure. It should be understood that additionalembodiments not limited to these particular features described areconsistent with the following examples.

Example 1

Using commercially available effusion cells (available from Applied Epi,http://www.appliedepi.com/), bulk superlattices containing Bi₂Te₃/TiTe₂layers were prepared by sequentially depositing layers of bismuth andtellurium, followed by titanium and tellurium layers. The layers weredeposited so as to provide the correct stoichiometric composition andabsolute amount of each element to prepare the targeted number of Bi₂Te₃and TiTe₂ layers, each layer being a unit cell. The deposition can becontrolled to produce any ratio of Bi₂Te₃ and TiTe₂ layers. Using thismethod superlattices having the repeating units [(Bi₂Te₃)₂(TiTe₂)₂],[(Bi₂Te₃)₃(TiTe₂)₃], [(Bi₂Te₃)₄(TiTe₂)₄], [(Bi₂Te₃)₅(TiTe₂)₅] and[(Bi₂Te₃)₆(TiTe₂)₆], [(Bi₂Te₃)₆(TiTe₂)₂] were prepared. The depositedprecursor was then annealed to kinetically trap the desired superlatticeproduct. Superlattices and superlattice precursors prepared according tothe present method were characterized by X-ray diffraction. XRR isuseful in describing the deposited layers after a deposition.

Another useful technique is EPMA, which is useful for determining theelemental composition of materials prepared using the modulatedelemental reactant technique. The precision of this instrument isusually within 1-2 atomic percent.

Example 2

This example describes the calibration process for deposition ofsuperlattice precursors. Calibrations are performed by making binarysamples with repeating layers of two elements. One element thicknessmust be kept constant, while the other thickness is varied. The samplesare then analyzed by XRD and EPMA.

FIGS. 9A, 9B and 10 illustrate the calibration process for MERdeposition. With reference to FIGS. 9A and 9B, samples were made bysystematically changing the layer thickness of bismuth while holding thelayer thickness of tellurium constant. Composition was determined byEPMA, and the ratio of layer thicknesses resulting in Bi₂Te₃stoichiometry was selected for synthesis of the subsequent superlatticeprecursors. Similarly, titanium-tellurium binaries were synthesized andanalyzed by EPMA to determine the thicknesses resulting in a TiTe₂stoichiometry. With continued reference to FIGS. 9A and 9B, once thethickness ratios in the binaries were determined, a series of sampleswith an approximate 1000 Å thickness was made by combining both binarysystems into alternating layers. A systematic series of these sampleswas made that changed the TiTe₂ thickness and left the Bi₂Te₃ thicknessconstant. Each sample of the series were made with an approximate 1000Å. The actual repeat layer thickness of the system (determined from XRR)can be plotted against the intended thickness of the varied component(TiTe₂) of the deposited material to give a thickness calibration of thesystem. This process was repeated until the desired thicknesses wereobtained. These plots can be seen in FIGS. 8, 10 and 11. Absoluteamounts of each binary multilayer were adjusted by analyzing this graphand preparing additional samples in the series of systematically variedsuperlattice precursors to determine the thickness of each componentlayer.

Example 3

This example describes an annealing study to determine the optimalannealing conditions for forming Bi₂Te₃/TiTe₂ superlattices. XRD can beused to monitor crystallization as a function of annealing temperature.XRD is characterized by the diffraction of X-rays that occurs within thelattice planes of a crystalline structure.

A [(Bi₂Te₃)₃(TiTe₂)₃] superlattice precursor was deposited using MER andanalyzed using XRD. The sample was then monitored via XRD as the samplewas annealed. A summary of the XRD results is recorded in FIG. 3. TheXRD study indicates that structural order perpendicular to the substrateincreases as a function of annealing temperature. Specifically, theBi₂Te₃ layers, each of which had a thickness of 10.0 A, yieldeddiffraction maxima at 2θ values of 8.8°, 17.6°, 26.4° and 44.0°, whilethe TiTe₂ layers, each of which had a thickness of 6.5 Å, yieldeddiffraction maxima at 2θ values of 13.6°, 27.2°, 40.8°and 54.4°. Withcontinued reference to FIG. 3, weak diffraction maxima are observed at40.8° and 54.4° in the initially deposited superlattice precursor, whichindicates the presence of small crystallites of Bi₂Te₃ and TiTe₂,respectively. As the sample is annealed, the characteristic peakscorresponding to the superlattice increase in intensity and resolution.However, the superlattice begins to disproportionate into itsconstituent binary compounds at about 300° C., and diffraction peakscorresponding to the phase separated compounds are observed in the 350°C. spectrum. The diffraction data resulting from the modulated nature ofthe precursors tracks the evolution of the sample with temperature andtime permitting the annealing conditions to be efficiently optimized.

Rocking curve scans also were collected as a function of annealingtemperature to monitor the evolution of interfacial roughness in themultilayer [from the (0 0 1) reflection] and the changes in thealignment of the crystallites that form from the (0 0 8) and (0 0 10)reflections (these areas in the XRD pattern depend on the layer spacingof TiTe₂ and Bi₂Te₃, respectively). The (0 0 1) rocking curve scanindicates little change in the diffuse scattering up to 240° C. Above240° C., the diffuse scattering becomes more intense, indicating thatthe interfacial roughness of the multilayer is increasing. The fullwidth at half maximum (FWHM) of both the (0 0 8) and (0 0 1 0) highangle rocking curves narrow steadily from 8 and 10 degrees,respectively, at 160° C. to 3.2 degrees at 280° C.

Example 4

This example describes the preparation of pellets of bulk superlatticeshaving the repeating unit [(Bi₂Te₃)₆(TiTe₂)₂]. Flakes of bulk[(Bi₂Te₃)₆(TiTe₂)₂] were prepared via MER deposition, followed byannealing at approximately 270° C. The annealing process was monitoredby XRD for the presence of the characteristic Bragg reflectionscorresponding to superlattice formation. After annealing several flakeswere combined in a mold and subjected to hot isostatic pressing under avacuum at 300° C. using a pressure of 700 MPa for 10 hours. Two ingotswere prepared according to this protocol, with the first (parallelingot) having dimensions of 2.013 by 8.030 by 2.058 mm, weighing 224.8mg and having superlattice Van der Waals gaps oriented parallel to thelongest dimension of the ingot. The superlattice orientation can beselected by placing the flakes in the mold in the desired orientation.The second (perpendicular) ingot had the dimensions 3.031×3.026×9.32,9.332, weighed 517.3 mg, and had Van der Waals gaps orientedperpendicularly to the longest dimension of the ingot.

XRD analysis of both pellets after hot pressing confirmed, by thepresence of characteristic Bragg reflections, that the superlatticestructure was retained. The thickness of each superlattice layer[(Bi₂Te₃)₆(TiTe₂)₂] was 76 Å, as calculated from Bragg's formula.

The two ingots were fully characterized with respect to theirthermoelectric properties. Several properties can be further optimized.For example, the perpendicular ingot had a Seebeck coefficient of about×47 μV/K and a resistivity of about 62 mΩ•cm at 300 K, whereas theparallel ingot had a Seebeck coefficient of about ×47 μV/K and aresistivity of about 1.2 mΩ•cm at 300 K. The resistivity of the parallelsample is comparable to that of Bi₂Te₃. However, the resistivity of theperpendicular sample is significantly higher. The Seebeck coefficientsfor both samples are about five-fold lower than that of Bi₂Te₃. However,both the resistivity and the Seebeck coefficient can be tuned by, forexample optimizing the number of carriers. The number of carriers can beincreased or decreased by doping with an appropriate material. Forexample, the Seebeck coefficient can be increased by decreasing thenumber of carriers. One known method for doping the present materialsincludes substituting a fraction of one chalcogen for another. Forexample, a percentage of the tellurium present in Bi₂Te₃ can be replacedwith selenium.

Both superlattice samples exhibited reduced thermal conductivity ascompared to the bulk materials. FIG. 16 compares the lattice thermalconductivity (κ_(L)) of the novel bulk superlattice materials, preparedas above, with bulk Bi₂Te₃ (sample C), which is a currently commerciallysuccessful thermoelectric material. The κ_(L) was measured lengthwise oneach sample. As a result, κ_(L) was measured parallel (sample A) andperpendicular (sample B) to the superlattice orientation. Thesuperlattice materials exhibit significantly lower κ_(L) than Bi₂Te₃ atlow temperature, and comparable κ_(L) at higher temperatures, such asroom temperature.

The present invention has been described with reference to preferredembodiments. Other embodiments of the invention will be apparent tothose of ordinary skill in the art from a consideration of thisspecification, or practice of the invention disclosed herein. It isintended that the specification and examples be considered as exemplaryonly, with the true scope and spirit of the invention being indicated bythe following claims.

1. A composition, comprising: a first layer comprising a material havinga high power factor; and a second layer comprising a diffusion barrier.2. The composition according to claim I where the material having a highpower factor has a formula Bi_(x)Sb_(2-x)Se_(y)Te_(3-y), orPbSe_(z)Te_(1-z) where 0≦x≦2, 0≦y≦3, and 0≦z≦1.
 3. The compositionaccording to claim 1 where the diffusion barrier comprises a materialhaving a formula ASe_(z)Te_(2-z), where A is selected from the groupconsisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and combinationsthereof, and 0≦z≦2.
 4. The composition according to claim 1 where thefirst layer comprises at least one of Bi₂Te₃, Sb₂Te₃, Bi₂Se₃, Sb₂Se₃,TiTe₂, HfTe₂, ZrTe₂, PbTe, TiSe₂, HfSe₂, ZrSe₂, PbSe, alloys thereof,and combinations thereof.
 5. The composition according to claim 1 wherethe first layer and the second layer are repeating layers forming asuperlattice.
 6. The composition according to claim 1 where the firstlayer and the second layer form a repeating unit.
 7. The compositionaccording to claim 5 where the first repeating layer comprisesBi₂Te_(3.)
 8. The composition according to claim 1 where the first layerincludes Bi₂Te₃, and the second layer includes TiTe_(2.)
 9. Thecomposition according to claim 5 where the first repeating layercomprises Sb₂Te_(3.)
 10. The composition according to claim 5 where thesecond repeating layer comprises HfTe₂, TiTe₂, or both.
 11. Thecomposition according to claim 5 further comprising a third repeatinglayer.
 12. The composition according to claim 11 where the thirdrepeating layer comprises a material having a formulaBi,Sb_(2-x)Se_(y)Te_(3-y), or PbSe₂Te_(1-z), where 0≦x≦2,0≦y≦3, and0≦z≦1.
 13. The composition according to claim 11 further comprising afourth repeating layer.
 14. The composition according to claim 13 wherethe fourth repeating layer comprises a diffusion barrier material. 15.The composition according to claim 13 where the fourth repeating layercomprises a material having a formula ASeTe_(2-z), where A is selectedfrom the group consisting of Ti, Zr, Hf. V, Nb, Ta, Cr, Mo, W, andcombinations thereof, and 0≦z≦2.
 16. The composition according to claim11 where each layer is from about 3 to about 200 Å thick
 17. Thecomposition according to claim 13 where the first, second, third andfourth layers comprise a repeating unit.
 18. The composition accordingto claim 13 where the first layer comprises Bi₂Te_(3.)
 19. Thecomposition according to claim 13 where the second layer comprisesTiTe_(2.)
 20. The composition according to claim 6 where the repeatingunit is from about 6 to about 500 Å thick.
 21. The composition accordingto claim 6 where the repeating unit is from about 40 to about 100 Åthick.
 22. The composition according to claim 11 comprising Bi₂Te₃,TiTe₂, and Sb₂Te_(3.)
 23. The superlattice according to claim 13 wherethe second and fourth layers comprise a material having a formulaASeTe_(2-z), where A is selected from the group consisting of Ti, Zr,Hf, V, Nb, Ta, Cr, Mo, W, and combinations thereof, and 0≦z≦2.
 24. Thecomposition according to claim 23 where each layer of the repeating unitcomprises at least one of Bi₂Te₃, TiTe₂, and Sb₂Te_(3.)
 25. Thecomposition according to claim 17 comprising a repeating unit having afirst layer including Bi₂Te₃, a second layer including TiTe₂, a thirdlayer including Sb₂Te₃, and a fourth layer including TiTe₂.
 26. A methodfor making a thermoelectric superlattice, comprising: synthesizing afirst material, the first material having a formulaBi,Sb_(2-x)Se_(y)Te_(3-y), or PbSe_(z)Te_(1-z) where 0≦x≦2, 0≦y≦3, and0≦z≦1; and synthesizing a second material on the first material, thesecond material being a diffusion barrier.
 27. The method according toclaim 26 where the second material has the formula ASe_(z)Te_(2-z),where A is selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta,Cr, Mo, W, and combinations thereof, and 0≦z≦2.
 28. The method accordingto claim 26 where the first material is synthesized by MER.
 29. Themethod according to claim 26 further comprising synthesizing a thirdmaterial, the third material having a formulaBi_(x)Sb_(2-x)Se_(y)Te_(3-y), or PbSe_(z)Te_(1-z) where 0≦x≦2, 0≦y≦3,and0≦z≦1.
 30. The method according to claim 26 where the second material issynthesized by MER.
 31. The method according to claim 29 furthercomprising synthesizing a fourth material, the fourth material being adiffusion barrier.
 32. The method according to claim 26 where the firstmaterial and the second material are synthesized as a repeating unit.